Novel Silicone Ceramer Coatings for Aluminum Protection

In: High Performance Coatings for Automotive and Aerospace… ISBN:978-1-60876-579-9 Editor: Abdel Salam Hamdy Makhlouf ©2010 Nova Science Publishers, Inc.

Chapter 1

NOVEL SILICONE CERAMER COATINGS FOR ALUMINUM PROTECTION

Atul Tiwari and L. H. Hihara Hawaii Corrosion Laboratory, Department of Mechanical Engineering,

University of Hawaii at Manoa, Honolulu, HI 96822, USA

1.0 INTRODUCTION Coatings encompass a relatively diverse class of materials and predominantly serve the

purpose of preventing metallic materiel from reverting back to their natural oxide state by corrosion. Studies coordinated by the German Research Society of Surface Treatment, the National Coating and Paint Association, and other independent agencies demonstrated the need for research and development on different coating materials [1-4]. Today, coatings are used to impart a variety of properties and characteristics to metal substrates such as corrosion resistance; aesthetics; antifouling, antibacterial, self-sensing, and self-cleaning abilities; wear and scratch resistance; thermal conductivity; UV resistance; etc. Moreover, coatings should ideally be easy to apply, be environmentally friendly, and allow for the incorporation of fillers for other desired characteristics.

Sol-gels, which are an interesting class of materials, have significant promise to meet many of the coating challenges of the 21st century. The sol-gel process, however, has been known for a very long time, and was first investigated in 1845 by Ebelmen [5]. The major thrust to this field was later provided by Roy [6]. Several recent and excellent reviews are available on sol-gel science [7-12]. The sol-gel process has drawn significant attention due to its advantages such as low cost, ease of reaction between different constituents, control over the final structure at nano-dimensional scales, and room temperature processing. The sol-gel process can be controlled and involves hydrolysis of alkoxy metal compounds followed by a condensation reaction and hardening process. One particular system, namely silicon-based sol-gel coatings, will be the focus of this chapter. Preparation of silicon-based sol-gel coatings begins with mixing alkoxysilane with water and a desired volatile solvent. Hydrolysis of alkoxysilane results into intermediate silanol (Si-OH) moieties that condense with the release of water molecules and the evaporation of the solvent. The silanol moieties further condense

Atul Tiwari and L. H. Hihara

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to form SiOSi polymeric chain macromolecules. The three-dimensional network formed as a result of crosslinking between the silanol moieties consists of a porous structure that condenses further at higher temperatures or with the passage of time, resulting in a dense ceramic material [13]. The sol-gel process also allows the synthesis of hybrid ceramic-polymer materials or ceramers by the combination of ceramic and polymers. Ceramers represent a network system prepared by a sol-gel reaction of functional organic polymers with a metal alkoxy compound. These materials are also called “Ormosil” when organic polymers reacts with alkoxysilanes [14].

This chapter focuses on prior work on ormosil or ceramer coatings, with the intent of informing the reader of the many aspects and facets of synthesizing, characterizing, and testing the coatings.

1.1 Background Corrosion of metals can be prevented with the use of protective coatings, which can be

either barrier, inhibitive, or sacrificial. The selection of a coating generally depends on the substrate and intended application.

The effectiveness of barrier coatings in preventing ingress and accumulation of moisture and corrosives at the substrate generally improves with better packing of polymeric chains, minimization of coating defects such as pinholes, and better adhesion to the substrate. Inhibitive coatings contain corrosion inhibitors to mitigate corrosion. One example is chromate conversion coatings that are very effective on aluminum, but may be banned in the future by the Environmental Protection Agency [15] due to health issues. These chromate conversion coatings are applied on aluminum using mixtures of hexavalent chromium salt and chromic acid. The oxidation-reduction reactions between the chromium compounds and the aluminum surface results in a thin film of insoluble trivalent chromium and soluble hexavalent chromium compounds. These hexavalent compounds leach out when the coating is breached and form insoluble trivalent chromium at the site of damage [16]. Sacrificial coatings contain active metals such as zinc or magnesium that acts as a sacrificial anode in a galvanic couple. Corrosion protection is afforded to the substrate as long as a sufficient amount of sacrificial coating is still present.

The ceramer silicone-based coating is primarily a barrier coating. The silicone resin combines the advantage of inorganic ceramics with that of organic polymers. Silicone constitutes a special class of coating materials that shows superior properties required for a wide variety of applications. Silicones are prepared by controlled hydrolysis of silane compounds that results in silanols and finally condenses to silicones. Silicones are frequently used as material of choice for different coating applications. Those that have alkyl or phenyl groups generally have desirable properties. For example, the ethyl group displays better flexibility, hardness, water repellency, and chemical resistance. These could be used when fast-curing rates, resistance to thermal shock, and low temperature application are desired. Phenyl-bearing silicones show good oxidative resistance and heat resistance as well as a longer shelf life and less thermo-plasticity compared to other members of the silicone family. However, properties of silicone coatings are affected by the amount of hydrocarbon they contain. A greater amount of organic hydrocarbons will increase flexibility, cure times, thermo-plasticity, and tackiness when cured in ambient conditions [17].

Novel Silicone Ceramer Coatings for Aluminum Protection

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Figure 1. Different applications of silicone clear coatings.

2.0 SYNTHESIS AND DEVELOPMENTS

Formulation of a coating consists of three vital components - the solvent, pigment, and

binder - which are crucial to curing and hardening. Solvents are usually a mixture of aromatic and aliphatic hydrocarbons, which control hydrolysis and prevent gelation. Pigments/fillers such as titanium dioxide are added to create whiteness as well as UV resistance; or to create redness, iron oxide or cadmium red is utilized. The binder and its curing process are very important in controlling the final properties of the coating. Most silicones cure completely in 60 min. at approximately 200 oC. However, the curing time and temperature can be reduced by adding an appropriate metal catalyst such as zinc or tin compounds. Silicone formulations are often combined with organic vehicles such as acrylic, epoxy, or alkyds to give them additional functionalities such as improved film forming capabilities, higher hardness, reduced thermo-plasticity, and faster curing times. The following subsections review different kinds of silicone coatings.

2.1 Silicone-Rich Coatings Coating formulations that have a silicone backbone without hydrocarbon are ideal glassy

materials for coatings. However, they are brittle and may crack, rendering a less than adequate performance. Proper selection of coating components is therefore needed for the development of effective barrier coatings.

Silicone-rich coatings can be prepared by adopting controlled hydrolysis followed by end termination with functional groups [18-20]. Polysilsesquixones are synthesized by acid catalyzed hydrolytic polycondensation of trimethoxymethylsilane, triisopropoxymethylsilane, or triisobutoxymethylsilane. Polysilsesquixones so obtained could be diluted with a volatile


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solvent for dip coating. However, the performance of the coating depends on the polarity and crystallinity of the substrate. Also, polycondensation in coating occurs between hydroxyl groups or between hydroxyl and alkoxy groups only by elevated-temperature curing [21, 22]. Hybrid coatings could be prepared without an acid catalyst by reacting γ-glycidoxypropyltrimethoxysilane (GPTS) and tetraethoxysilane in ethanol using hexamethylenediamine (HMDA) and 3-aminopropyltrimethoxysilane (APTS) as a crosslinker, followed by the addition of a dibutyltindilaurate-hardening catalyst and ultrapure water [23].

An oligomeric polyfunctional sol-gel precursor that is obtained by combining cyclic siloxane with chlorosilane followed by hydrolysis or an ethanolysis process can be used to create a low molecular-weight monomeric compound that results in a dense silicone hybrid coating [24]. Such a coating provides good UV protection, improved abrasion resistance, and enhanced stability against acid compared to a traditional automotive clear coat. Barrier properties of 3-GPTS pretreatment on copper and aluminum alloys followed by powder coating of epoxy-polyester show that silane-treated substrates provide better corrosion resistance than floreo-zirconate-treated substrates [25]. Barrier properties of a water-based silane mixture prepared by reacting vinyltriacetoxysilane with bis-(treimethoxypropylsilane) amine display good adhesion to 2024Al-T3 and 6061-T6 Al surfaces. The precursor to the coating forms stable covalent bonding with the aluminum surfaces [26].

2.2 Coatings via Silicone-Polymer Blending Silicones can be coupled with epoxy resin to create an interpenetrating coating system in

which hydroxyterminated polydimethylsiloxane acts as a modifier, γ-aminopropyltrimethoxysilane (γ-APTMS) as a crosslinker, and dibutyltindilaurate as a catalyst. Polyamidoamine and aromatic polyamine adducts can be used to cure the coating. Thermal properties of such siliconized epoxy coating systems are higher than that of the unmodified epoxy coating systems. Morphological studies show heterogeneity in the siliconized epoxy coating, a characteristic that was amplified as the silicone concentration in the epoxy network increased [27]. Siliconized epoxy coatings can also be prepared using hydroxyterminated polydimethylsiloxane as a modifier, γ-APTMS as a crosslinker, and dibutyltindilaurate as a catalyst. Polyamidoamine, aromatic amine adducts, and phosphorous-containing diamines are used as catalysts in such a case. The corrosion and fouling resistance behavior of such siliconized epoxy coatings when evaluated using potentiodynamic polarization, electrochemical impedance spectroscopy, salt spray, and antifouling tests exhibit a lower corrosion current and higher paint film resistance than does the pure epoxy coating [28].

A coating suspension for improved scratch resistance of a polymethylmethacrylate surface was prepared by reacting silatrane with 3-GPTS in the presence of an acid catalyst. Scratch resistance of the coated surface increased with the increase in the alkoxysilane content in the coating. It is important to note that the curing time and temperature affects the scratch resistance and adhesion properties of the coating layer [29].


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2.3 Nano- and Molecular Composite Coatings Nano-composite coatings have at least one of the components with dimensions less than

100 nm. Such coatings show a high degree of crosslinking in the network structure due to the presence of nano dimensional modifiers [30-32]. Such nano particulates have a large aspect ratio (surface/volume) for reaction and can form strong bonds with other components, resulting in a stronger and tougher material with fewer defects [33]. Molecular composites on the other hand consist of a flexible moiety dispersed in a rigid macromolecular network. Rigid nano particulates can provide support to the flexible macromolecular unit, and can impart desired characteristics for specific applications.

Sol-gel hybrid nano-composite coatings can be prepared by evaporation of solvent and induced partitioning of phases. Several hybrid polymer layers self-assembled to form strong covalent bonds between organic-inorganic moieties, resulting in an optically-transparent coating, displaying increased indentation hardness (i.e, 1.0 GPa), corresponding to a dense silica film. This hierarchical nano-composite coating displayed an oriented, nano-laminated under-layer bonded to an isotropic worm-micellar top-layer [34]. A scratch-resistance coating was developed by incorporating nanoparticles into a polymeric matrix. For example, methacroyloxy-propyl-trimethoxysilane modified nano-sized silica and alumina particles were used as fillers in a polyacrylate matrix nano-composite coating [35]. Silica nanoparticles can be fabricated via controlled hydrolysis of TEOS in the presence of acid. The size of silica nanoparticles depends on the duration of the hydrolysis reaction. Silica nanoparticles of different particle sizes were fabricated by functionalizing them with 3-(trimethoxysilyl)propyl methacrylate. The methoxysilane group terminated the particle growth and stabilized the nanoparticles [36].

Incorporation of uniformly-distributed nanoparticles may enhance thermo-mechanical stability of the resultant nano-composite coatings. For example, uniform distribution of nano-SiO2 enhance the thermal stability of an acrylic nano-composite coating, while an agglomeration of these nanoparticles in the polymer deteriorated the properties of the coatings. Formation of the Si-O-Si network by evenly distributed nano-SiO2 contributes to the anti-oxidation process, char-accumulation, and stable char architecture [37]. Similarly, a super hydrophobic surface was obtained by using a coating material that has low surface energy and adequate roughness at the micro/nano meter scale. A silicone nano-filament coating was prepared using an equimolar amount of trichloromethylsilane and water vapor displaying a high water contact angle (~higher than 150o) on coated substrates, an indication of the super hydrophobic nature of the coating [38]. Similarly, water-based, hybrid nano-composite coatings were prepared that showed good mechanical as well as other unique properties [39].

Molecular composites having a low dielectric constant, polyimide rigid phase, coupled with flexible polysiloxane phases, can be obtained using a solution-blending technique. Partly miscible coating compositions displayed uniform distribution of polysiloxane microspheres in a continuous polyimide matrix. The low surface energy of polysiloxane occupied an entire surface of the molecular composite coating that hindered the in-diffusion of water molecules. Such molecular composite coatings has the potential to effectively reduce corrosion in microelectronic and other electronic devices [40].


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2.4 Coating Containing Fillers and Pigments Active functionalities of fillers and pigments could modify characteristics of the coating.

A hybrid coating was prepared by reacting tetraethoxysilane, tetrabutylorthosilane, tetrabutyorthotitanate, and hydroxylterminated polydimethysiloxane. The tetrabutylorthotitanate filler affected specific surface areas, pore size, and pore volume, but did not affect pore morphology. Specific surface area and pore volume decreased with the decrease in tetrabutylorthotitanate [41]. Similarly, a flexible hybrid material was obtained by reacting polydimethylsiloxane with alkoxide of zirconium and tantalum. It was discovered that the inorganic component in the material was in the form of oxide clusters and attached to polydimethylsiloxane through metal-oxygen bonds [42].

In another study, phenyltrimethoxysilane, 2-(3, 4-epoxycyclohexyl)trimethoxysilane, and tetramethoxysilane were reacted in ethanol. Hexamethylenediamine was used as crosslinker while zincacetate, carbon nanotubes, and Nanosil silica nanoparticles were used as nano-fillers. A small amount of water was added to the formulation to induce a hydrolysis reaction and dibutyltindilaurate was used as a catalyst. The reactive sol formulation was left overnight to allow homogenization. This coating provided excellent corrosion protection of aluminum in chloride-containing atmospheres and solutions [23]. Another coating was synthesized by the reaction of polydimethylsiloxane, titaniumtetraisopropoxide, and ethylacetoacetate along with variable amounts of colloidal silica. The hardness of the coating remained proportional to the silica filler content and curing temperature, and inversely proportional to the concentration of polydimethysiloxane. Hydrophobicity was determined by measuring the water contact angle that increased with the increase in polydimethylsiloxane content in the coating. However, the hardness value decreased with the increase in polydimethylsiloxane concentration in the coating [43].

A sol-gel composition was obtained by reacting a mixture of titania nanoparticles in an epoxy polymer with glycidoxypropyltrimethoxysilane (GPTS) and TEOS. An increase in the TiO2 concentration decreased the rate of polymerization while the epoxy group conversion was induced — effects that may have been caused by the UV absorption competition between the TiO2 nano-particles and the photo initiator. An increase in the concentration of dispersed TiO2 gave rise to an opaque coating, while a transparent film was obtained when TiO2 nano-particles were generated in-situ in the presence of a suitable coupling agent. Also, the UV photo curing of these coatings resulted in a homogeneous dispersion of inorganic particles into the organic matrix without macroscopic phase separation [44].

Organic corrosion inhibitors are also used in sol-gel organosilicate hybrid coatings. Coatings with incorporated organic corrosion inhibitors were examined by potentiodynamic polarization to investigate the inhibition activity of the entrapped compounds. Scanning vibrating electrode and electrochemical impedance techniques determined the effectiveness of corrosion inhibitors in the coating. Incorporation of several organic inhibitors in the coating reduced the corrosion on aluminum substrates [45].

The effect of cerium salt, 3-mercapto-propyl-trimethoxysilane, and octadecyl-trimethoxysilane treatment was investigated. It was discovered that treating the metal substrate with cerium salt did not improve the corrosion resistance. However, treating the substrate with a hydroalcoholic solution or octadecyl-trimethoxysilane was effective in reducing corrosion. Moreover, treating the substrate with 3-mercapto-propyl-trimethoxysilane had less effect than in the former case [46]. The self-healing capability of surface


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pretreatments formulated from cerium-nitrate doped (triethoxysilylpropyl) tetrasulfide silane solutions was evaluated. Precipitation of cerium oxides and hydroxides in anodic areas inhibited corrosion. The inhibitive mechanism of cerium-doped silane provided better corrosion resistance than did the base coating. However, the optimum/critical concentration of cerium nitrate in the coating appeared important in controlling corrosion [47]. Sol-gel coatings composed of dimethoxydimethylsilane, methyltriethoxysilane, and tetrapropoxyzirconium with an added organic corrosion inhibitor tertachloro-p-benzoquinone were prepared. Electrochemical impedance spectroscopy, atomic force microscopy, and glow discharge optical emission spectroscopy determined that increasing the concentration of the organic inhibitor did not provide enough corrosion protection due to the disorganization of the sol-gel system. However, lowering the concentration of the inhibitor resulted in homogenous coating structures with increased corrosion inhibition characteristics [48].

A coating formulation was synthesized using an inorganic salt in the sol of tetramethoxysilane, organic polymeric additives, and well-dispersed chromium oxide fillers. SEM, differential scanning calorimetry, and X-ray diffraction patterns demonstrated that chromium oxide increased the flexibility of the tetraethoxysiloxane-derived ceramer coating [49].

2.5 Effect of Catalyst in Coating Formulations Methyltriethoxysilane and tetraethoxysilane were reacted in the presence of an acid

catalyst to form a sol-gel coating [50]. An increase in the Si-CH3 bond concentration increased the overall thickness of the coating. Fourier Transform Infrared (FTIR) spectroscopy and ellipsometry established a correlationship between the methyl content and microstructure in the coating. Methyltrimetoxysilane and 3-GPTMS with 3-APTMS and N-(2-aminoethyl)-3-aminopropyltrimethylsiloxane reacted in the presence of an acid catalyst to form a corrosion resistant coating. FTIR spectroscopy confirmed the molecular structure of the coatings. Atomic force microscopy showed the surface morphology did not reveal surface structures, suggesting a very dense packing arrangement within the molecular chains. Results from various analytical techniques showed that the optimum concentration of the hardener was the key factor in controlling the corrosion resistance over aluminum alloys [51]. Dibutyltindilaurate catalyst played an important role in the synthesis of the tetraethylorthosilicate sol-gel system. Various analytical techniques used on the gel and solid coating indicated that lauric acid derived from the hydrolysis of dibutyltindilaurate contributed significantly to modifying the condensation path of the Si moiety. Lauric acid acted as a catalyst for the hydrolysis and condensation of silicones. FTIR and small angle X-ray analysis indicated the possibility of hetro-condensation between Sn and Si moieties [52].

2.6 Factors Affecting the Coating Process The following section reviews some critical factors in the coating process. Aside from the

synthesis of coating formulations, there are other factors that can affect the coating quality.


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2.6.1 Surface Preparation The adhesion of a coating is highly dependent on the condition of the substrate surface,

and, therefore, surface cleaning and preparation play a vital role in the coating process. In aluminum alloys, for example, the formation of an Al2O3 layer is dependent on time and temperature (Fig. 2), and, hence, the surface condition would be dependent on prior processing and heat treatments. Since the surface bonding with coatings may be dependent on the oxide layer, surfaces generally should be etched or abrasive grit blasted to remove the existing oxide layer to obtain consistent results.

Figure 2. Thickness growth of inert aluminum oxide layer over aluminum surface as a function of time and temperature. Adopted from Ref. [53] and reproduced after permission from Elsevier Publishing.

It should be noted that due to the high reactivity between aluminum and oxygen, a thin passive oxide layer will immediately reform on the aluminum surface after cleaning. The abrasive grit-blasting technique will also introduce surface roughness that may be beneficial to adhesion for some coatings.

2.6.2 Pretreatments

Surface pretreatments are often used to enhance bonding. The interaction of silicones with inorganic metal surfaces depends on the nature and preparation of the surfaces prior to coating. For example, the amount of hydroxyl groups available over an aluminum surface for the reaction with coating functionalities depend on pretreatment conditions [54]. Some examples of surface pretreatments that have been used are summarized below.

Adhesion was enhanced between aluminum and an epoxy coating through a porous pseudoboehmite oxyhydroxide layer (Fig. 3) that was formed by hydration of the aluminum substrate in boiling water [55]. A thin layer resulting from phosphate treatments promoted the adhesion of silicone coatings on aluminum [56]. In another case, silicone coatings were applied to a tacky epoxy primer layer to ensure the in-diffusion of silicones into the epoxy matrix that acted as a coupling agent between the silicone coating and metal substrate [57]. A few chromium-ion based primers such as strontium chromate were also used as a pretreatment [16, 58]. Review on surface treatment can be found in reference [59].


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Figure 3. TEM image of a cross section of epoxy-psuedoboehmite-aluminum layers. Thin hairs of a pseudoboehmite oxyhdroxide layer, which facilitate the strong bonding, are clearly visible. Image acquired from [55] and reproduced after permission from Elsevier Publications.

2.6.3 Method of Application Coatings can be applied by various techniques such as spraying or dipping, which can

affect the coating thickness and hence its properties. Base-catalyzed particulate sols were used to prepare silica hybrid coatings. A crack-free

hybrid silica coating results from either electrophoretic deposition or dip coating in a mixture of methyltrimethoxysilane and ethytriethoxysilane containing sodium hydroxide. Corrosion resistance improved when the coating thickness was more than two microns. The barrier properties of the coating obtained by eletrophoretic deposition were superior to that achieved with dipping [60].

2.6.4 Drying and Curing

The drying and curing processes involve a hydrolysis reaction followed by condensation and polymerization of monomers (Fig. 4). In the early stages, particles grow by nucleation followed by agglomeration and network formation that finally leads to gelling. Rapid drying of the gel at elevated temperatures may 1) introduce residual stresses in the coating network, resulting in cracking, or 2) lead to the formation of pores and cavities. Also, when curing at elevated temperatures, the mismatch in coefficient of thermal expansion between the substrate and coating may also lead to delamination. Slow heating or ambient temperature hardening is preferred to enable the gradual release of volatile organic components minimizing pore formation. Drying and hardening processes in coatings can be analyzed using spectroscopic techniques [61].

The effects of drying and curing temperature on GPTS-coated oxidized aluminum substrates was studied. Changes in the curing temperature induced significant changes in the final coating morphology. Coating thickness increased with the increase in temperature, possibly due to a higher crosslinking density introduced between the silanol groups. [62].


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Figure 4. Plot of coating viscosity as a function of time during the application and hardening process. Reproduced from Ref. [63] after permission from Marcel Decker Publications.

3.0 CHARACTERIZATION OF SILICONE COATINGS

The development of novel ceramer coatings relies on extensive material characterization to gain a thorough understanding of coating synthesis, curing, and performance. Various analytical techniques used in research and development of silicone ceramer coatings are summarized below with examples from various studies. The information is not intended to be a comprehensive guide, but rather a collection of work showing how others have used the techniques in their research.

3.1 Fourier Transformation Infra-Red Spectroscopy FTIR can be used to study coating composition as well as to monitor the reaction process

as a function of time. Several available FTIR techniques such as attenuated total reflection [64-66] for coatings hardened on a metal surface, diffuse reflectance [67] for ultra-thin films, and transmittance [68, 69] for clear or transparent coatings are useful. FTIR is useful for determining parameters such as residual porosity and molecular bonding. If used in concert with a microscope, excellent spatial resolution can be obtained.

Rapid kinetics or in-situ observation of the curing reaction mechanism using time-resolved FTIR spectroscopy [70] can provide valuable information about the volume collapse or hardening route in the complex ceramer network. The residual porosity Vp in the coating structure can be determined by FTIR analysis using the following equation [71]:


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dpV α

α−=1

while, dd

Ax α=; and αα .. xxA dd == .

Here xd and αd (1.0 x 104 cm-1for dense silica film) are the film thickness and the

absorption coefficient of the dense film, respectively, while x and α are the film thickness and the absorption coefficient of a porous film; A is absorbance.

The effect of polytetrahydrofuron in TEOS hybrid film was studied by FTIR analysis, which displayed a defect band at 560 cm-1 assigned to the skeletal vibration of 4-fold siloxane rings [72]. The intensity if this band was a function of the polymer content and molecular weight increment. Changes in the deconvoluted bands at 1080 cm-1 suggested that porosity of the hybrid film depends on the polymer content and average molecular weight. Polytetrahydrofuraon hinders the reactivity of the silanol groups and retains the 4-fold fold siloxane rings in the hybrid gel film that leads to porous structure compared to the pure silica film.

FTIR was used to monitor the hydrolysis of phenylaminomethyl trimethoxysilane (PAMS) and variable concentrations of TEOS [73]. The band at 1167 cm-1 corresponding to SiOSi recorded progress of the hydrolysis reaction. Formation of H3O+ ion increased the hydrolysis of alkoxysilanes while OH- ion formation increased the condensation between different hydroxyl functionalities. The area ratio of bands at 600 cm-1 and 1070 cm-1

monitored the amount of cyclic species forming in the solution at a given time. The rate of hydrolysis increased in solutions containing pure TEOS compared to those having a mixture of TEOS and PAMS. An increase in the pH of the solution due to PAMS dissociation and steric hindrance from the bulky groups in the mixture lowered the rate of the hydrolysis reaction. Moreover, the formation of cyclic species increased in pure PAMS solution but decreased in the case of mixed silicones, probably due to the reaction of TEOS with PAMS before it could react each other to form cyclic. The change in the amount of cyclic species, polymerization rate, and functionalities of the precursor changed the gelation time of the precursors.

FTIR spectra recorded on ceramer gel as a function of time is shown in Fig. 5. Peaks appearing at approximately 2900 cm-1 corresponding to symmetric and asymmetric –CH2 stretching [74] were found consistent and independent of time. Si-O-Si vibrations appeared at about 1080 cm-1 [75]. Spectral assignments indicated that the hydrocarbon portion of the composition did not take part during the initial phases of the reaction. Free and hydrogen-bonded hydroxyl stretching appeared in the region of approximately 3500 cm-1 [74]. The intensity of the stretching decreased as the temperature increased, probably because the water molecules de-bonded from amine or hydroxyl groups in the coating. It is likely that these water molecules and those obtained from the condensation of silanol groups participated in the hydrolysis of the alkoxy groups in silanes. A possible kinetic pathway can be estimated by monitoring the time-dependency of the peak heights at 1350 cm-1 and 1950 cm-1, which literature shows corresponds to hydroxyl/amines and Si-O bond stretching modes [76, 77],


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respectively. The drop in intensity of other peaks in the spectra could be attributed to various condensation and cross-linking reactions occurring simultaneously.

Figure 5. Ambient conditions FTIR analysis of silicone ceramer coating [78].

3.2 Raman Spectroscopy

Raman spectroscopy has been particularly useful in studying the chemical changes that

occur during the conversion of solution to gel, gel to glass, and glass to ceramic [79-82]. If used in concert with a microscope, excellent spatial resolution can be obtained.

The hydrolysis reaction of tetraethoxysilane (TEOS) that contained variable amounts of water molecules was recorded with Raman spectroscopy [83]. Bands at 650 cm-1 corresponding to TEOS, and at 600 cm-1 corresponding SiOSi were used to monitor the hydrolysis reaction. The intensity of the band at 650 cm-1 decreased with the increase in H2O/TEOS ratio in the mixture. Band intensity at 600 cm-1 increased with the reduction intensity of 650 cm-1 band, suggesting the hydrolysis and subsequent condensation of silicones. GPTS and APTS systems displayed a doublet at 643 cm-1 corresponding to SiO3 symmetric and anti-symmetric stretching vibrations of GPTS [84]. Intensity of these bands decreased with the increase in time of hydrolysis. Bond intensities were observed at 881cm-1 and 1035 cm-1 corresponding to CCO- stretching vibrations from ethyl alcohol and CO- vibrations from methanol suggesting the formation of alcohol during the hydrolysis of silane (Fig. 6). The hydrolysis reaction velocity decreased at elevated temperatures due to a decrease in the pH of the medium. The pH dropped due to the neutralization of acidic silanol groups. The insertion or presence of hydrocarbon between the silicon atoms could be investigated by looking into the peaks at 1350 cm-1 and 1030 cm-1 that appears due to the scissoring and wagging mode of –CH2- groups, respectively [85].

Aluminum coupons coated with a quasi-ceramic silicone coating were exposed for 4 months at Mauna Loa Observatory on the Big Island of Hawai‘i that receives high levels of solar radiation. In another experiment, a similar set of coated coupons were exposed for 60 hours in artificially-simulated sunlight. No visible damage was detected after the prolonged exposure periods. Raman spectra recorded before and after the exposures indicate that the coatings were unaffected after 60 hours of intense UV radiation in the lab, while coatings exposed for 4 months at the outdoor site showed distinct changes in structural patterns. The structural changes in the coatings could be due to coating degradation or to extended reactions


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occurring between different chemical entities as a result of prolonged exposure to solar radiation [86].

Figure 6. Raman spectral assignments of the GPTS/APTS hybrid material. The spectra were recorded after 0 min. (dotted line), 15 min. (dashed line) and 14 hours (solid line) after the start of the sol-gel reaction. Plot acquired from ref [81] after permission from Springer Publication.

3.3 Nuclear Magnetic Resonance Spectroscopy

The reaction mechanism between different chemical entities in a polymeric system can be

effectively studied using Nuclear Magnetic Resonance Spectroscopy (NMR) [87-89]. Analysis can be conducted on the coating precursors as well as on the solidified coating.

Organic silicones such as glycidoxypropyltrimethoxysilane tend to undergo a ring-opening reaction in acidic or basic conditions to form diol and other hydroxyl compounds. Such reactions can be effectively monitored with NMR technique [90].

GPTMS-TEOS ormosil system displayed following 13C peaks in NMR analysis [91]. 8.0-9.4 ppm [Si-CH2], 22.0-23.4 ppm [Si-CH2CH2CH2], 73.2-74.2 ppm [Si-CH2CH2CH2-O-CH2], 71.4-72.0 ppm [Si-CH2CH2CH2-O-CH2], 51.2-52.3 ppm [Si-CH2CH2CH2-O-CH2CH[O]CH2], and 44.2-45.2 ppm [Si-CH2CH2CH2-O-CH2CH[O]CH2].

Similarly, following 29Si NMR peak assignments has been reported [92, 93]: -49.4 to -50.7 ppm [T1= R-Si(O)2(OH)], -54.6- to -59.9 ppm [T2=R-Si(OR)(OH)2], -65.1 to-68.8 ppm [T3=R-Si(OH)3], -92.8 to -94.0 ppm [Q2=Si(OSi)2(OH)2], -102.7 to -103.4 ppm [Q3=Si(OSi)3(OH)] and -111.1 to -113.0 ppm [Q4=Si(OSi)3].

Moreover, 17O NMR signals have been reported (Table 1) for hydrolyzed pure dimethyldiethylsiloxane (DMDES), methyltriethoxysilane (MTES), and tetraethoxysilane


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(TEOS), as well as their reaction products [94]. Here D refers to bi-functional Me2SiO2 units, T to tri-functional units and Q to tetra-functional SiO4 units.

Table 1:17O NMR assignments for different alkoxy systems. Reproduced [94] with permission from American Chemical Society.

Sample Chemical shift (ppm) Assignment

DMDES 33.7 D-OH 63.9, 67.5 D-O-D

MTES 29.7 T-OH 56.9 T-O-T

TEOS 15.3 Q-OH 31.0 Q-O-Q

DMDES/TEOS 15.0 Q-OH 47.5 D-O-Q

64.0, 67.5 D-O-D

DMDES/MTES 31.7 D-OH / T-OH 60.4 D-O-T 64.1 D-O-D

MTES/TEOS

15.4 Q-OH 30.3 T-OH or Q-O-Q 44.7 T-O-Q 57.1 T-O-T

NMR analysis shows that water in the synthesis process is used for hydrolysis as well as

an epoxy ring opening. However, when less water is used, it is preferably consumed in the hydrolysis process. Hydrolysis with large amount of water leads to a product that displayed poor corrosion inhibition probably due to the formation of a less well-connected network structure. Poor silicone-particle packing and the formation of channels could allow the percolation of corrosive media to the substrate. Moreover, the formation of hydroxyl-bearing groups due to the ring opening of epoxide group induces hydrophilicity in the coating structure, which attracts electrolyte into the coating, promoting corrosion. Coatings containing low hydrocarbon content that were synthesized with low water content during hydrolysis displayed better packing arrangement and barrier properties than those containing higher hydrocarbon content.

Similarly, 29Si and 13C NMR studies showed that metal alkoxide such as titanium alkoxide play a vital role in alkoxysiliane formulations and enhanced the degree of condensation. A higher degree of condensation in titanium alkoxide and availability of secondary condensable entity enhances the reactivity in such sols. Titanium alkoxide also facilitates the ring’s opening of an epoxy group. For example, in the case of GPTS and titanium ethoxide formulations, there was as much as 78% ring cleavage. However, an excess of water is required for ring cleavage and hydrolysis since the reactivity of titanium alkoxide with water is low. Ether-bond formation between titanium alkoxide and epoxide rings was only possible when a sufficient amount of titanium hydroxide was present in the sol. Pre-existing ether linkages in the titanium sol was probably responsible for such anomalous behavior [90].


15

3.4 X-Ray Photoelectron Spectroscopy XPS is a versatile tool and provides both elemental and chemical information. Surface

survey scans, dedicated element scans, and elemental mapping can be done using the XPS technique [95, 96]. For example in Fig. 7, the deconvoluted Si2p signal reveals pure silicon at 99.69 eV, a small amount of Si2O at 100.64 eV, a Si2O3 moiety at 102.72 eV, and a weak SiO2 peak at 103.67 eV [97].

Figure 7. Si2p de-convolution of high resolution XPS analysis of silicone. Adopted from Ref. [97]

XPS analysis was conducted on the solid ceramer coatings to confirm the reaction pathways adopted by precursor macromolecules. Fig. 8 shows a surface survey scan obtained on the 6061Al-T6 coupon treated with a silicone coating. The deconvolution of Si2p spectra shows only one peak at 102.63 eV, which is due to 10.6 at.% silicon. The C1s spectra show four distinct peaks. The peak at 285.96 eV corresponds to a long carbon chain having 30.7 at. %, while that at 286.51 eV represents carbon joined to nitrogen (2.9 at.%) in aminosilane. Another peak positioned at 288.13 eV corresponds to carbon attached to different oxygen atoms (17.0 at.%), while the peak at 289.28 eV corresponds to an ester-type linkage (COOR ~1.1 at.%) of carbon. Looking to N1s spectra indicates nitrogen joined to carbon in aminosilane at 400.22 eV and nitrogen bonded to aluminum metal at 402.39 eV. These findings suggest that the backbone chain of aminosilane was intact and amine functionality reacted with surfacial hydroxyl groups of aluminum metal resulting in permanent covalent bonding between the coating and metal substrate. The O1s deconvoluted spectra show three peaks. The peak at 530.71 eV could be explained by Si-O-Al linkage, while the strong peak at 532.56 eV was probably due to an oxygen atom involved in the Si-O-Si linkage. An oxygen atom attached to a metal, probably SnO2, explains the weak peak at 534.07 eV [23].

Surface contamination and the cause of coating delamination could be effectively studied using the XPS technique. Silicone adhesives when utilized with organic-film adhesion may undergo oxidative degradation. The degraded byproduct could remain on the metal surface or diffuse into the organic film leading to delamination [98]. In grit-blasted aluminum and


16

GPTS-treated aluminum, both coated with an epoxy-base adhesive, XPS showed that chemisorption that was covalent in nature was the mode of adhesion for both substrates. However, on the GPTS-coated aluminum surfaces, covalent bonds formed between the curing agent and epoxy rings from GPS or the epoxy resin, while on grit-blasted aluminum, covalent bonds formed between different components of the adhesive [99].

Figure 8. XPS analysis of silicone ceramer coating. Adopted from Ref. [23] and reproduced after permission from Elsevier Publications.

3.5 Secondary Ion Mass Spectroscopy

Secondary Ion Mass Spectroscopy (SIMS) utilizes a high-energy primary ion beam to

sputter and eject secondary ions from solid surfaces. The ejected ions are analyzed with a mass spectrometer. Elemental and chemical information can be obtained. SIMS also has excellent spatial resolution.

Grit-blasted aluminum, as well as grit-blasted and GPTS pre-treated aluminum, were coated with a commercial epoxy adhesive. The reactions between the different coating components were analyzed using SIMS [99]. Several different hydrocarbon fragments were found at low mass; for example, C2H5

+ (mass/charge (m/z)=29 daltons (Da)), C3H5+ (m/z=41

Da) and C3H7+ (m/z=43 Da). These ions were contributions from the substrate as well as

GPTS coating. The fragments at m/z=28 Da were assigned to the presence of Si+ as well as a small contribution from C2H4

+. Similarly, other fragments at m/z= 77, 91, 128, and 178 Da originated from the phenyl group in the epoxy resin. Ions with even mass numbers indicate nitrogen molecules with an odd number of nitrogen atoms as per “Nitrogen Rule” suggested elsewhere [100]. It was realized that fragmentation from the cross-linked structure was difficult to compare with the uncross-linked material. Also, a covalent bond was formed


17

between the oxidized aluminum and a silanol group from hydrolyzed GPTS. The covalent bonds were formed between amine-silanol-epoxy groups when epoxy adhesive was applied over GPTS [99]. Permanent bonds of this nature are highly desirable as they are not susceptible to attack from corrosive media, including water.

The SIMS technique was used to analyze reactions and interaction in a silicone ceramer coating hardened over 6061Al-T6 aluminum alloy. Although SIMS only samples the uppermost mono-layers, reactions between the coating and aluminum substrate could still be reflected in the spectra due to mixing and in-diffusion of reaction products during the curing process, when the coating was still in the liquid state. A positive ion static SIMS spectrum is shown in Fig. 9. The spectrum displays a majority of CH fragments from a long chain hydrocarbon present in the coating backbone. A NH2

+ fragment from aminosilane is visible at m/z=16 Da, suggesting that there was still some aminosilane left in the coating that had not reacted and was presented as hydrogen-bonded moiety. Another peak at m/z=15 Da was from a CH3

+ fragment from a hydrocarbon portion of the chain. An additive effect from Al+ and C2H3

+ fragments could explain the high intensity of signal appearing at m/z=27 Da. Several different peaks such as Si+ (m/z = 28 Da), SiH+ (m/z = 29 Da), SiC+ (m/z = 41 Da), SiCH2

+ (m/z = 42), SiCH3

+ (m/z = 43 Da), SiO+ (m/z =44 Da), and SiOH+ (m/z =45 Da) appeared due to the fragmentation of silicones [101-103]. The signal at m/z = 56 Da corresponds to a SiOC+ fragment from methoxysilane, indicating the likelihood of the incomplete hydrolysis of the alkoxy group. The high intensity of the signal at m/z=18 Da (from water molecules) caused all other signals to appear relatively weak [23].

Figure 9. Positive ions SIMS spectra of silicone ceramer coating.

3.6 X-Ray Diffraction

X-ray diffraction (XRD) is a technique that can be used to analyze crystal structure,

chemical composition, and physical properties of coatings and their substrates. This technique utilizes the impingement of an X-ray beam of a known wavelength onto the sample under analysis. The X-rays are then diffracted by the crystal lattice giving a pattern of peaks of various intensity as a function of the diffraction angle. Materials with a crystalline structure


18

generate sharp peaks; whereas, amorphous materials may generate broad and ill-defined peaks. Results from XRD may also be used to extract various forms of information.

The clear presence of pure silica (as SiO2) may not be easily detected in hybrid coatings due to confinement or encapsulation of it by organic polymer chains. Therefore, XRD patterns obtained on such ceramer materials display a single homogenous amorphous phase without any evidence of the silica filler. However, the absence of sharp peaks in the XRD pattern and the presence of a broad peak at 21° 2θ value was evidence that amorphous silica was present with homogeneity in the sample, assuring good adhesion between inorganic and organic moieties [104]. XRD analysis also showed that the broad peak width (Fig. 10) centered at approximately 22o (2θ value) changed with the organosilicone concentration. Increase of polyorganosilicone in the composition shifted the diffracted peak to a lower angle, indicating the increase in inter-atomic distance in silicone [105].

X-ray diffraction has been indirectly used to obtain other information. For example, the diffusion of ions from the metal substrate into the coating has been studied [106]. A silica coating was applied over a copper substrate, and the in-diffusion of atmospheric oxygen through the silica coating resulted in the formation of copper oxide at the silica-copper interface. Diffusion coefficient values have been calculated using this method. The D value of 10-19 cm2/s has been determined for silica glasses at 1000oC, and 10-3 cm2/s at room temperature for largely porous silica gel glasses [107]. Calculations using room-temperature diffusion coefficient values obtained from the XRD technique suggested that the metal oxide layer may start building at the coating-metal interface after approximately 10 years of the exposure to the atmosphere [106].

Figure 10 XRD pattern of 3-(methacryloxypropyl)-trimethoxysilane hybrid silica powder with different mole ratios of TEOS/MEMO in the sols after heat treatment at 150 oC for 5 hours showing an amorphous coating network. Reproduced after Ref. [105] with permission from Elsevier Publications.

4.0 THERMAL ANALYSIS

The durability of barrier coatings can to some degree be related to the thermal stability of

the coating. Polymer chains that resist breaking up into smaller fragments as a result of heat


19

exposure will likely have more stable long-term mechanical properties—averting or delaying failure caused for example by increasing brittleness. It is therefore advantageous to analyze the stability of the coating spanning a range of temperatures. Such studies can provide information on coating durability in aggressive climatic conditions [108]. The following subsections summarize research related to the thermal analysis of silicone ceramer materials. The information is not intended to be a comprehensive guide, but rather a collection of work showing how others have used thermal techniques in their research.

4.1 Thermogravimetry Thermogravimetric analysis (TGA) records degradation (weight loss) of material as a

function of time and temperature. The onset of the decomposition temperature from the TGA curve suggests the thermal stability of the material while the ultimate residue that is obtained after the complete pyrolysis reaction gives the percentage of volatile or decomposable constituents in the compounds under investigation. Derivative thermogram (DTG) showing the rate of mass change is constructed from the weight loss curve as a function of time and temperature. Observations from the DTG give the decomposition profile and percentage of constituents decomposing at that particular time and temperature.

DTG of methyltriethoxysilane and TEOS-based hybrid material shows that the decomposition temperature of Si-CH3 increases with increasing tetraethoxysilane concentration. This suggests that a dense inorganic network hinders the out-diffusion of decomposed products and thereby increases the overall thermal stability of the material [109]. The organic-inorganic hybrid containing nitrogen, phosphorous, and silicon in epoxy resin was investigated. The hybrid system was compared to pure epoxy resin. The residue obtained after complete decomposition was higher in the hybrid materials due to the synergistic effect of nitrogen, phosphorous, and silicon in the system. Also, the activation energy from the decomposition of the hybrid material was higher than that of pure epoxy, proving that the hybrid material had higher thermal stability than that of pure epoxy [110].

Interaction between tetraethoxysilane and polyols was studied using thermal analysis, FTIR, and solid state NMR spectroscopy. Thermal analysis demonstrates that the polyols were bound to the silica matrix through chemical, as well as hydrogen bonding [111]. Thermal behavior of epoxy-polyester powder coating containing YiO2 and SiO2 particles was investigated. The TG/DTA-GC/MS technique revealed that benzene and phenol were formed as a result of thermal decomposition. Also, the thermal stability of the cured coating was higher than that of the resin. Reticulation of resin chains was responsible for the higher thermal stability of the coatings compared to their parent compounds [112]. Thermal stability and corrosion resistance of a hybrid coating prepared by reacting 3-GPTS and amino-terminated siloxane with tetraethoxysilane was determined. Spectroscopic techniques characterized the different forms of silicone in the coating structure. Deconvolution of the DTG curve showed a four-step decomposition pattern under a nitrogen atmosphere. The thermal stability of the 3-GPTS-TEOS hybrid and the activation energy required its decomposition were higher than that of the polydimethylsiloxane-TEOS hybrid. The finding suggests that the degradation of the aliphatic n-propyl segment was enhanced by the higher thermal conductivity of silica [113].


20

TGA was conducted on an organic/inorganic hybrid of a SiO2-matrix composite prepared by a sol-gel process utilizing tetramethoxysilane and TEOS as a precursor. Thermal stability of the hybrid material was enhanced due to the presence of tetramethoxysilane. A pure silica network was detected due to the thermal degradation and evaporation of the incorporated organic groups above 500 oC [105]. The thermal degradation kinetics of the methacryloyl group containing poly(methylphenylsiloxane) was studied using TGA. Friedman, Flynn-Wall-Ozawa, Coats-Redfern, and Phadanis-Deshpande methods were used to study the kinetics of degradation. They show that the first degradation stage of the poly(methylphenylsiloxane) material followed a three-dimensional diffusion mechanism, while the second degradation stage followed a nucleation and growth mechanism [114].

Ceramer coatings can be pyrolyzed at variable heating rates. The gases evolved as a result of material degradation can be analyzed using FTIR [115]. For example, Fig. 11 show TGA and FTIR of a solidified silicone ceramer coating at a heating rate of 20 oC/min in an inert atmosphere. The DTG plot suggests that material decomposed in four steps and that the final residue was 53%. The results indicate that material consists of at least 47% decomposable constituents that may evolve during the lifetime of the coating. The FTIR analysis of the evolved gas suggests that either the hydrocarbon or lower members of silicones were evaporating during the early stages of heating. The regime from 3026 to 2992 cm-1 shows symmetric and asymmetric vibrations from hydrocarbons. Strong peaks in the region beyond 3500 cm-1 correspond to the cleavage and decomposition of species containing hydrogen-bonded hydroxyl and amine functionalities. Furthermore, the hydrocarbon portion of the coating decomposed during several different heating windows. A detailed study of silicone ceramer coating decomposition and its kinetics has been conducted by Tiwari and Hihara [116].

(a) (b)

Figure 11. Weight loss of solid silicone ceramer coating as a function of time in inert atmosphere pyrolytic conditions displaying multiple steps decomposition processes (a). Evolved gas FTIR analysis of solid ceramer coating at 20 oC/min heating rate (b).

4.2 Differential Scanning Calorimetry

This technique records the amount of heat flowing between a sample and a reference

under controlled thermal conditions. The DSC curve reflects transitions that are related to


21

specific heat, the glass transition temperature, exothermic reaction peaks for crystallization or crosslinking reactions, narrow endothermic peaks for fusion or melting, broad endothermic peaks related to decomposition, and volatilization, as well as dissociation or increase/decrease in heat flow due to oxidation or thermal decomposition [108].

The thermal properties of a polysiloxane-poly(tetrafluroethylene) semi-interpenetrating coating system was investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. Thermal stability of the coating system increased with the increase in the poly(tetrafluroethylene) content. Each blend had a single glass transition temperature (Tg), suggesting that the blends were comprised of a single phase. Also, transitions corresponding to the semi-IPN network were observed. DSC can reveal distribution of components at the molecular level [117]. For example, silicone containing a tri-functional epoxy monomer was mixed with conventional epoxy resin and then analyzed using the DSC technique. A single glass transition temperature was observed indicating that all the components in the material were in a single homogeneous phase. Moreover, low Tg values were recorded due to an increased free volume in the composition that had a high amount of phenyl rings in the composition [118].

A PDMS-modified epoxy coating displayed two transitions in a DSC thermogram. A peak appearing at -46 oC corresponds to the melting of the PDMS segment while Tg was seen at -16 oC. The value of Tg was found dependent on the mobility of the polymer chains. Higher Tg values were observed for stiffer samples that restricted the mobility of the chains [119].

4.3 Dynamic Mechanical Analysis Dynamic Mechanical Analysis (DMTA) is a useful technique for determining the visco-

elastic properties of coating materials. This technique helps determine the glass transition temperature (Tg) that appears as an alpha relaxation as well as other relaxation processes (e.g., beta, gamma, and delta). The visco-elastic response of a material can be measured as a function of applied periodic load in the form of fixed frequency or resonant frequency. The oscillatory applied load could be in the form of tension, compression, flexural, or torsion that provides information on storage modulus, loss modulus, or damping values [120, 121].

The phase morphology of a silicone biocidal coating was determined using DMTA (Fig. 12). A single glass transition temperature was obtained from the Tan δ (represents the damping property of the material and defined as the tangent of the phase angle and the ratio of loss modulus over storage modulus) vs. temperature curve of the coatings, suggesting that there was no phase separation in the material. A higher value of a well-defined rubbery plateau region was seen in the amine-cured coating containing cyclic siloxane compared to those having linear siloxanes. Two clear transitions were seen in polybutadiene-cured siloxane biocidal coatings, indicating the incompatibility of silicones with polyalkanes. The presence of long hydrocarbon chains in siloxane acted as a compatibilizer that induced miscibility with the polyalkanes. However, microphase separation occurred due to an incompatibility between the two polymers. The modulus as well as the glass transition temperature were found to vary as a function of chain length in all coating formulations [122]. In the case of the polysiloxane-polyurea coating, the storage modulus (a measure of elastic response of the material different from Young’s modulus) was independent of temperature


22

before Tg and decreased after Tg was reached [123]. Tg was obtained from α-relaxation, and the storage modulus was independent of the coupling-agent concentration. However, the cross-linking density increased with the coupling-agent concentration. Also, the cross-linking density was found dependent on the TEOS content.

(a)

(b)

Figure 12. DMTA plot of coatings showing variation of storage modulus as a function of temperature (a). Tan δ as a function of temperature (b). Reproduced after Ref. [122] with permission from Taylor & Francis Publications.

5.0 MECHANICAL PROPERTIES

Mechanical properties such as modulus, hardness, tensile strength, compressive strength,

etc. can affect the durability of coatings. Conventional testing techniques can be used to obtain mechanical properties from self-standing coatings, while advanced techniques are required to evaluate coatings that are bonded to substrates. Two important mechanical tests are discussed in the following subsections.


23

5.1 Peel Test The peel test is useful in evaluating the adhesive strength between a coating and

substrate. A basic peel test involves peeling a flexible strip of thickness “h” and width “b” by applying a force “F” at an angle “θ” (Fig. 13). If residual stress is neglected and the peel arm is infinitely stiff, then the total energy per unit area “G” dissipated in de-bonding the strip is given as follows: [124]

)1( θCosbFG −=

The true adhesive toughness “GA” is given as:

)( opA RGGG −=

where “Ro” is a local radius of curvature, and “GP”is the plastic bending. The value of “GP” depends on the properties of the strip and conditions at the point of bonding.

Figure 13. Schematic of basic peel test employed in coating technology. Reproduced after Ref [124] with permission from Taylor & Francis Inc.

In one example, delamination occurred at the epoxy-silicone interface for an epoxy under-coated, duplex siloxane -butyl acrylate styrene silicone top coat system. Also, peeling took place due to the nucleation of cavities that grew until the coating failed. Moreover, it was discovered that the force required to peal the silicone-polymer duplex top coat from the epoxy undercoat decreased as the top and bond coats were made thicker [125].

An epoxy coating was peeled off of a polydimethylsiloxane-coated surface and a de-cohesion mechanism was recorded during the release of the epoxy film. Different peeling mechanisms were observed as a function of the thickness gradient and average coating thickness. For thin films with low thickness gradients, the peel mechanism was attributed to the formation of voids and their coalescence that started from the thinner section of the coating and proceeded to the thicker sections. In thicker coatings, peeling progressed radially-


24

inwards from the outer edges beginning in thinner sections and propagating to thicker sections. The formation of finger-like structures in certain regimes and peeling in thicker sections were also a mode of failure in some cases [126].

The Scotch® tape peel-off test was conducted on silanized 2024 aluminum, coated with a commercial silicone adhesive coating. XPS and FTIR analysis were utilized to study the silicone primer layer after the peel-off test. The primer layer remained integrated while failure occurred due to loss of adhesion between the commercial silicone adhesive and the silane primer [127]. Peel tests are very common in the automotive paint industry and are frequently used to analyze the strength of pressure-sensitive adhesives. Additives such as polysiloxanes and titanium dioxide are used in such adhesives and play an important role in formulation. Polysiloxanes are commonly used as an ingredient in clear coats and dramatically affect the performance of the coating system [128].

5.2 Nano-indentation and Nano-scratch Tests Thin reactive coatings that are bonded to metal surfaces can be tested by using nano-

indentation or nano-scratch techniques. Nano-indentation involves pressing an indenter of known geometry into the coating surface with a known load such that it penetrates through the coating layers. Hardness and modulus values of the coated materials can be determined by knowing the load and area under the indenter impression [129]. Especially for the case of thin coatings (e.g. <1.0 µm); nano-indentation is among the few known techniques that can be used to accurately assess the mechanical properties of the coating. Nano-indentation experiments can be designed to investigate the properties of coatings, substrates, and coating-substrate interfaces. The hardness Hc of a coating is given as a ratio of maximum load F and contact area A. The geometry of the nano-indenter and the contact depth hc can be translated into the contact area. The elastic modulus of coating Ec could be determined using the initial slope S if the unloading curve is such that Er=(πS/2A)0.5, where Er is the reduced modulus given as

i

i

c

cr EE

E22 11 νν −

+−

= .

Here Ec, νc and Ei, νi are the elastic modulus and Poisson’s ratio of the coating and

indenter, respectively. Nano-mechanical coating properties can be influenced by the coating substrate, and,

therefore, it is recommended to use indentation data when the indenter-penetration depth is only 10% of the coating thickness. When a coating is subjected to an indentation experiment, several different modes of damage may occur as the load on the indenter increases. For example, the coating may delaminate due to a loss of contact between the coating and its substrate, and brittle coatings may fracture. These failure modes can be used to determine the fracture strength, toughness, or residual stresses of the coating network and its interface with the substrate.

Hybrid materials that contain higher organic branching points for cross-linking reactions displayed higher hardness values compared to linear polymers [130]. In most cases,


25

meaningful results are derived from penetration depths less than 10% of the coating thickness to exclude the effect of the substrate [131]. The effect of AlOOH boehmite nano-particles and nano-rods in a 3-GPTMS nano-composite coating was investigated using the nano-indentation technique. Modulus and hardness values for nano-particles that filled coating were 9.88 GPa and 0.98 GPa, respectively, while that of nano-rods that filled coating were 8.86 GPa and 0.83 GPa, respectively. The alignment of nano-rods in the coating resulted in lower mechanical properties [132]. Reduced modulus and reduced hardness varied through the different layers of the coating [133]. Higher-reduced modulus and reduced-hardness values were recorded at the surface due to a condensed morphology; lower values were recorded in the bulk due to a porous structure, and values increased again at the coating-metal interface due to a denser structure [133, 134].

In hybrid coatings, compositions with higher organic content tend to cause segregation in the network, resulting in high-silica regimes surrounded by hydrocarbon-rich regimes. These coating networks display distinctly different mechanical properties than the pristine silica network does. A silica-rich coating based on TEOS displayed near-elastic behavior while a hybrid GTMS coating displayed increased penetration on loading and almost complete recovery during unloading (Fig. 14). The TEOS coating showed the smallest amount of creep due to a densely-packed rigid silica network while creep was high for the GTMS coating due to the viscoelastic flow and relaxation processes associated with long-chain hydrocarbon-rich domains. Young’s modulus also decreased with the increase in organic portion in the backbone of the coating structure. In fact, the modulus of GTMS was 25 times less than that of TEOS [135].

Figure 14 This creep analysis shows load-displacement curves using a spherical indenter for tetraethoxysilane, methyltrimethoxysilane, vinyltrimethoxysilane, and GTMS coatings over a silicon wafer. Reproduced [135] with permission from Elsevier Publishing.


26

Nano-mechanical properties were determined on quasi-ceramic, high-silicone content coatings that were aged for three months on three different aluminium alloy substrates. Hardness and modulus values were determined as a function of displacement into the coatings (Fig. 15). The average hardness values for the quasi ceramic-coated 2024Al, 6061Al, and 7075Al were 0.42 GPa, 0.41 Gpa, and 0.47 GPa, respectively, while the average modulus values were 4.40 GPa, 4.51 Gpa, and 5.45 GPa, respectively. The loading-and-unloading curves (Fig. 16) demonstrated the elastic recovery of the coatings with negligible plastic deformation [136].

Figure 15. Nano-mechanical analysis and surface morphology of quasi ceramic-coated aluminium alloys. Hardness and modulus values of the hardened coating on three different alloys as a function of displacement into the surface. Reproduced [136] with permission from Elsevier Publications.

The average hardness of uncoated alloys 2024Al, 6061Al, and 7075Al were 1.87 GPa, 1.47 GPa, and 2.37 GPa, respectively, while the average modulus were 77 GPa, 76 Gpa, and 78 GPa, respectively. The slight variation in the mechanical properties of the quasi-ceramic coatings on different surfaces suggested that the coating may have been influenced by a small degree from the substrate mechanical properties or by solubilized alloying elements from the substrate alloy.


27

Figure 16. Load applied on quasi-ceramic coated three different alloys as a function of displacement into the surface showing the elastic recovery in the coating network.

In a nano-mechanical scratch test, cracks grew in a silicone ceramer coating on a 6061Al substrate due to the propagation of the indenter head. The coating cracks (Fig. 16) were perpendicular to the direction traversed by the indenter head. The length of the cracks increased in proportion to the force on the indenter and to the penetration of the nano-indenter through the thickness of the coating. Delamination and peeling occurred (Fig. 17) when the load on the indentation head exceeded the force required to hold the molecular segments in the coating. The results indicated that the degree of plastic deformation in the coating was limited, which is characteristic of the highly cross-linked coating [23].

(a) (b) (c)

Figure 17. Scanning electron micrograph of mechanically-damaged silicone ceramer coating. Images of initiation of indentation (a), propagation of indenter head (b), and near termination of indentation (c).

Coatings applied on substrates can display lower mechanical properties with elevated temperature-curing if ions from the substrate diffuse and accumulate in the coating, modifying its structure. Residual stress values suggest that the coatings are put in tension


28

when the substrate restricts coating contraction due to shrinkage. Additionally, the higher coefficient of thermal expansion of the coating compared to the substrate contributes to more residual tensile stresses on cooling. For coatings that contain photo-curable groups, curing with UV light has resulted in compressive residual stresses, suggesting these types of coatings expand on curing [137].

6.0 MORPHOLOGY Silicone coatings containing active hydroxyl groups gradually condense over time,

resulting in a dense three-dimensional structure. These high-density coating networks can be investigated with electron and scanning probe microscopy

6.1 Electron Microscopy Scanning electron microscopes (SEM) reveal the surface morphology of coatings. High-

resolution SEM or field emission SEM is capable of scanning dense ceramer coating surfaces. Field emission scanning electron microscopy (FESEM) analysis of a coating on 7075Al

and cured for 15 days displayed different layers on the coating. The top layer was dense while the succeeding layers showed clusters of sol particulates (Fig. 18a). The surface topography was investigated for a super-condensed quasi-ceramic coating (Fig. 18b). Nanometer level surface roughness was seen on the coated surface probably due to the nature of the condensed sol [136].

(a)


29

(b)

Figure 18. FESEM images: a cross section of a ceramer coating over an aluminium substrate (a); condensed surface topography of quasi ceramic coating displaying nanometer resolution (b).

High-resolution micrographs of nano-scale silicone coating containing TiO2 nano-particles showed that silicone not only engulfed pigment particles, but also provided reinforcement to the coating structure along with nanometer-scale porosity [138]. SEM of pristine silica colloid displayed un-agglomerated spherical colloid particulates with 20 nm diameters. Slight porosity was also observed due to random packing of the colloid that gave rise to gaps between the particles. The addition of GPTS polymer filled the gaps between the particles, and the addition of larger amounts of GPTS resulted in a featureless topography due to smooth siloxane coverage [139].

The SEM can also be used to study the corrosion morphology of coated substrates. For example, a ceramer-coated 2024Al-T3 specimen was studied under SEM after potentiodynamic experiments in different corrosive solutions [140]. Bare samples showed extensive pitting corrosion while very little pits were observed on the coated specimens. Severe pitting was observed in a region of a coating defect (Fig. 19a). Active pit growth caused coating delamination (Fig. 19b).

(a)


30

(b)

Figure 19. SEM image of pit formation in crack of sol-gel coating (a); morphology of corrosion pit on the surface of coated aluminium alloy (b). Reproduced [140] with permission from Elsevier Publication.

6.2 Atomic Force Microscopy

Atomic Force Microscopy (AFM) is a technique that can be used to study the topography of coatings under high-spatial resolution. Phase image and surface topography of the sample can be acquired with nanometer surfacial resolution using the tapping-mode AFM technique [141].

Three different interpenetrating siloxane networks modified with polytetraflouroethylene, polyolefin, and acrylic resin were studied with AFM [142]. Cylindrical shaped grains were observed on pure silicone coatings, while spherical grains were observed on coatings containing 20 wt. % PTFE. A knitted structure was observed in the sample with an 8.5% polyolefin modifier suggesting that the degree of interaction of silicone with different polymers varies and results in distinctly different phase morphologies.

Silicone ceramer and quasi-ceramic coatings were coated over aluminum and analyzed using the AFM technique. The nanographs of silicone ceramer coating displayed heap/lump-shaped structures with dimensions in nanometers, which probably formed from an agglomeration of gel particulates that ultimately condensed into a ceramic-like phase. It is known that an increase in an organic component can lead to reduced connectivity between the silica-gel networks, thereby producing increased flexibility in the ceramer backbone. The incorporation of an organic component reduced the condensation reaction rate and increased in gelation time, which in turn resulted in a dense ceramer structure. Fig. 20a shows an AFM image of the silicone ceramer coating obtained after the complete hardening profile. It appears from the nanoscopic image that silicone macromolecular (polymer) chains present in liquid phase passed through a gel phase as they were exposed to ambient conditions. Finally, the particles in the gel phase condensed to form a solid bundle-like morphology as shown in the nanograph. These ceramer particulate bundles froze on the surface of the metal due to possible electronic interactions between silicone and the metal surface. An atomic force microscope image of the surface roughness of the quasi-ceramic coating on an aluminium substrate is shown in Fig. 20b. The average particle size was approximately 70 nm, while the average height of the heap (formed as a result of sol particulate confinement) was on the order


31

of 130 nm. The surface roughness appeared in the nanograph includes the roughness that already existed on the bare and mirror finish polished aluminium surface. A dense coating network without any signs of holidays was seen under the scan [136].

(a) (b)

Figure 20. A three-dimensional surface topographical AFM image. Arrangement of sol particles in hardened silicone ceramer coating over aluminum alloy (a); nano-structures in a silicon quasi ceramic coating on an aluminum alloy (b).

6.3 Contact Angle

A surface is hydrophobic if the contact angle between it and a liquid droplet is large.

Studying the contact angle therefore provides information on the wettability of a coating. Water-based saline solutions absorb on the coating surface via hydrogen-bond formation. Salt ions and water molecules may then propagate through interconnecting coating defects and reach the substrate. If the coating-substrate bonds are destroyed, the coating will delaminate from the substrate. Hydrophobic surfaces are therefore required to discourage the settling of liquid over the coating surface. Studying the contact angle and surface tension reveals the interactions between solids and liquids. Such interactions are pivotal in understanding the adsorption, absorption, spreading, adhesion, washability, and wettability of a liquid on a coating.

The contact angle θ is defined as the angle that liquid forms at the liquid-gas-solid interface (Fig. 21). The angle θ can be found using Young’s equation [143].

⎟⎟⎠

⎞⎜⎜⎝

⎛

γγ−γ

=θV,L

L,SV,SCos

Where γS,V, γL,V and γS,L are the surface tension (or surface energy) of the solid/vapor,

solid/liquid and liquid/vapor interfaces, respectively. If the angle is less than 90o, the liquid will wet the solid; if it is greater than 90o, the liquid will not wet the solid. A contact angle higher then 150o is a sign of a super-hydrophobic surface.


32

Figure 21. Forces acting on a liquid droplet over a solid surface.

Contact angle measurements provide direct evidence of surface hydrophobicity in silicone-coated aluminum substrates [127, 144]. Water repellency and impermeability of GMPTS- and MTMS-modified waterborne polyurethane coatings were determined by measuring the contact angle. A value of 80±3o was measured for a pristine silicone sol-gel coating, while a value of 48±4o was measured for a pristine polyurethane coating. The contact angle of the polyurethane was increased to 58±2o by modifying it with silicone [145]. A transparent hydrophobic hybrid silicone nano-composite film was synthesized consisting of TEOS, GPTS, and isobutyltrimethoxysilane. Silica nanoparticles were added to increase the roughness on the coated surface. The contact angle with water was measured for different compositions and a high 130o value, which was obtained for a 7 nm thick coating [146]. In an ormosil coating, the contact angle ranged from 74.9o to 97.9o, and increased with the hydrocarbon chain length [147].

7.0 ELECTROCHEMICAL AND CORROSION ANALYSES

The electrochemical and corrosion behavior of a material can be investigated in different

ways, with each technique providing a partial picture of the corrosion process. Cathodic and anodic polarization techniques artificially polarize the material from its steady-state corrosion potential. Anodic dissolution characteristics and cathodic reactions kinetics can be examined. For a more realistic measure of corrosion behavior, exposure, or immersion, experiments can be conducted where the specimen is allowed to corrode at its natural open-circuit corrosion potential. Weathering experiments introduce other degradation parameters such as the effect of ultraviolet and solar radiation on coating degradation.

7.1 Cathodic and Anodic Polarization Polarization of a coated metal electrode can provide insight on the barrier strength of the

coating and propensity to induce cathodic reactions. If the coating has low dielectric strength, it may break down during the polarization process, which induces high voltage gradients in


33

the coating. If the coating resists dielectric breakdown, it can prevent or attenuate metal dissolution during anodic polarization [148].

An Al2024-T3 specimen coated in-situ with 35 μm zeolite (microporous crystalline aminosilicate) had an anodic polarization current density of 10-8 A/cm2 in various corrosive media (i.e. 0.5 mol/L H2SO4, 0.5 mol/L NaCl/HCl), indicating good barrier properties of the coating [149]. In comparison, the anodic current densities for bare Al2024-T3 in similar corrosive media are approximately between 10-2 to 10-4 A/cm2. Polarization experiments in a 0.6 M NaCl solution were also conducted on bare Al2024-T3, and Al2024-T3 coated with a mixture of bis-(trimethoxysilylpropyl)amine and vinyltriacetoxysilane [26]. While the general shape of the polarization diagrams were similar for the bare and coated specimens, the current densities of the coated Al were significantly lower than that of the bare Al, indicating that the coating acted as a physical barrier and was not reduced or oxidized during polarization.

The protection efficiency (P) of a coating can be calculated based on the corrosion current density of coated and bare substrates [51].

⎟⎠⎞

⎜⎝⎛ −

=corr

ocorr

ii(%)P 1100 ,

where io

corr and icorr represent the current density of bare and coated electrodes, respectively. Polarization experiments were performed on silicone-ceramer coated Al6061-T6

electrodes in aerated and de-aerated 0.5M Na2SO4 solutions at various scan rates [23]. The open-circuit potentials of the bare 6061Al-T6 became significantly more positive for the aerated solutions; whereas, those of the coated specimens were relatively unaffected by aeration (Fig. 22). These results indicate that the coatings are non-conductive or non-catalytic to oxygen reduction. The open-circuit potentials for the coated specimens are likely to be controlled by hydrogen evolution due to similar open-circuit potential values in both de-aerated and aerated solutions. The anodic current densities of coated electrodes are significantly lower than that of the bare 6061Al-T6 electrodes near the open-circuit potential, an indication that the silicone-ceramer serves as an effective barrier coating near the open-circuit potential at which corrosion would normally occur. The lower current densities are likely due to a lower electric field through the passive alumina layer on the aluminum surface due to the presence of a silicone-ceramer coating. Another contribution to the lower current densities could be a deficiency of O2- anions needed for the growth of the passive film. For the bare electrodes, O2- can be supplied by H2O in the electrolyte; whereas, for the coated electrodes, O2- must originate from the coating itself or by the eventual diffusion of H2O to the substrate surface from the electrolyte through the coating. At higher potentials, the anodic current densities of the coated electrodes approach and ultimately converge with that of the bare 6061Al-T6. This could be caused either by H2O permeating the coatings at higher potentials, stripping of O from the coating surface, or dielectric breakdown of the coating.


34

Figure 22. Anodic polarization diagrams of uncoated and coated (with silicone ceramer) 6061Al-T6 in aerated (left), and de-aerated (right) 0.5 M Na2SO4 At 30°C. Scan rates of 1.0 and 0.1 mV/s were used. The similarities between the scans conducted at 1.0 and 0.1 mV/s suggest that the anodic current density is a function of potential and not exposure time.

7.2 Exposure Tests

The efficacy of a barrier coating can be assessed by designing laboratory exposure

experiments such as immersion tests in various salt solutions, or accelerated atmospheric exposure tests where humidity and other parameters are controlled.

Two different 1050Al plates treated with an epoxy primer and an elastomeric silicone topcoat were immersed in real sea water for 7 months. One of the top-coated plates also had an intermediate epoxysilane layer. The coating on the plate without the intermediate epoxysilane layer delaminated during the immersion experiments; whereas, the coating on the plate with the intermediate layer survived. This experiment indicates that the intermediate epoxysilane layer was necessary for the adhesion of the elastomeric silicone coating to the epoxy primer [57]. Bare and silicone-ceramer coated 2024Al-T6, 6061Al-T6, and 7075Al-T6 aluminum alloys were also immersed in Harrison’s solution for 720 hours at room temperature in the open-circuit condition. The bare specimens showed severe corrosion; whereas, the specimens coated with ceramer coating were immune to corrosion, except on some sections of the specimen edges where the coating had chipped. Higher coating thicknesses at the specimen edges may have increased residual stresses in those regions. The lack of corrosion in the planer regions of the specimens indicates that silicone-ceramer coatings were impervious to the solution and had adequate adhesion to the substrate, demonstrating their excellent barrier properties [23].

In another experiment, silicone-ceramer coated 2024Al-T6, 6061Al-T6, and 7075Al-T6 aluminum specimens were immersed in 3.15 wt% NaCl solution for 2,160 hours. The coated specimens were generally free of corrosion, except on the edges where some of the coating had chipped off. Samples were periodically removed from the immersion bath and examined. Slight corrosion activity was seen on the coated 2024Al specimens after 720 hours; few corroded regions appeared on the coated 7075Al specimens after 2,160 hours; and no corrosion or surface degradation was seen on the coated 6061Al specimens after 2,160 hours of immersion [136].


35

Silicone-coated as well as -pretreated 2024Al-T3 and 6061Al-T6 coupons showed comparable and in some cases better corrosion protection than chromate-treated coupons when exposed to the ASTM B117 test. Silicone-pretreated alloys were top-coated with polyester or polyurethane and subjected to an ASTM 1654-92 related experiment that required scribing the coupon. The silicone-pretreated and polyester or polyurethane top-coated coupons performed well even after 1,008 hours of the salt spray test, and no delamination was seen at the scribed site. Similarly, silicone pretreated and polyester or polyurethane top-coated coupons performed well in the ASTM B368 and Machu test [26].

Various 2024Al-T6 coupons coated with ormosils having short and long hydrocarbon chain lengths were exposed in the ASTM B117 test for 168 and 672 hours. After 168 hours of exposure, the 2024Al-T6 coupons coated with ormosils having shorter chain hydrocarbons displayed localized pitting due to primary film failure; whereas, ormosils with longer chain hydrocarbons did not. For either type of ormosil, no coating delamination was seen during this exposure period, but coating degradation increased after 672 hours of exposure. The corrosion resistance of the coating increased with the hydrocarbon chain length [147].

Barrier properties of a quasi-ceramic nano-coating were examined in accelerated corrosion tests. Fig. 23 show Video Image Enhanced Evaluation Weathering (VIEEW) scans of the coupons that were retrieved after a cyclic corrosion test chamber experiment conducted as per GM9540P standards. Three aluminum alloy coupons (i.e., 2024Al, 6061Al, and 7075Al) were coated and exposed in the cyclic corrosion test chamber for 192 hours. One coated coupon from each set was scratched to study the effect of corrosion at the coating-metal interface. For comparative purposes, uncoated coupons of the same aluminum alloys were exposed simultaneously. The uncoated coupons suffered severe surface damage due to corrosion, while the coated coupons were not affected. Slight corrosion at the edges of the coated coupons was probably due to the coating process rather than the coating itself. Scribed coupons corroded in the scratched area, but there was no sign of coating delamination adjacent to these scribed sites [136].


36

Figure 23 Surface scanning of the uncoated (UC) and quasi-ceramic coated (C) aluminum coupons after 192 hours in an accelerated corrosion experiment. Uncoated coupons suffered severe surface damage, while the coated coupons showed very little corrosion. Coated and scribed coupons showed slight corrosion in the scribed region, with no undercutting.

7.3 Accelerated Weathering

It is difficult to accurately predict the service life of a coating using accelerated corrosion

tests. Other factors such as solar radiation may also degrade coatings, leading to corrosion of the substrate. Coatings containing organic groups can degrade and fail prematurely when exposed to natural solar radiation. Ultraviolet (UV) radiation from visible light is responsible for the initiation of photolysis of covalent bonds, and infrared (IR) radiation induces thermo oxidative degradation [150, 151]. It is therefore important to consider sunlight exposure when estimating coating life. Designing realistic weathering tests for real applications may involve many other variables (Fig. 24).

Suitable photostablizers such as TiO2 were introduced in a quasi-ceramic coating to render it more resistant to photo-induced degradation and failure [136]. Aluminum coupons were coated with the coating, and then exposed in simulated sunlight in a Q-Panel Q-Sun XE3 Xenon arc test chamber for 60 hours at 70 oC, and exposed at a relatively dry, high-altitude, high-solar-radiation test site for four months. The Raman spectra did not change for the specimen exposed to the simulated sunlight, but significant differences were observed for the specimen exposure at the high-altitude test site. The lack of changes in the Raman spectra for the specimen exposed to simulated sunlight may have been due to the short exposure period. The changes in the Raman spectra for the specimen exposed outdoors indicated that chemical changes in the coating may have resulted from UV radiation in sunlight.


37

Figure 24. Variables encountered in weathering tests. Reproduced after Ref. [152] with permission from Elsevier Publications.

8.0 MICROBIAL AND ANTI-FOULING CHARACTERISTICS

Ships and marine structures are in constant contact with corrosive seawater that is also

home to marine organisms. Organic coatings have been the material of choice for corrosion protection in the ship-manufacturing industry. Effective marine coatings should have good mechanical properties and a dense molecular structure to resist the in-diffusion of sea water [153], as well as possess anti-fouling properties. Environmental concerns imposed by the EPA have encouraged research and development of novel coating materials containing non-leachable environmental friendly biocides. Two different approaches have been adopted towards the development of biocidal coatings. One approach involves covalently attaching a nonmetal biocide to the backbone of a polymer; while the other approach involves synthesizing low surface energy polymers with anti-fouling characteristics.

The anti-fouling properties were studied for a coating with incorporated biocidal triclosan that was covalently bonded to a siloxane backbone through different alkyl chains. This biocidal siloxane coating displayed a significant reduction in macro-fouling characteristics when the modulus of the coating was between 0.1 and 10 MPa. When the modulus was too low, barnacles cut through the coating, and when the modulus was too high, the biocide may have been trapped in the highly cross-linked polymer. [122]. Similarly, anti-microbial coatings of n-halamine siloxane were prepared, and then exposed to range of viral, fungal, bacterial, and yeast micro-organism suspensions to test their anti-fouling properties. The coatings displayed a broad range of anti-microbial properties and killed most of the microbes in contact within a small period of time. Moreover, the coatings prevented the foul ammonia odor which is normally generated by bacterial activity [154, 155].

The leeching rates of sodium benzoate and benzoic acid from silicone coatings were compared to investigate their effectiveness as anti-fouling and anti-bacterial agents. Benzoic acid formed large crystals inside the coating and leached out quickly, while sodium benzoate formed smaller aggregates and leached slowly suggesting better long-term effectiveness.


38

Enhanced anti-bacterial and anti-fouling characteristics were seen in the silicone coating containing the sodium benzoate [156].

A silicone quasi-ceramic coating was applied to three different aluminium alloys (i.e., 2024Al, 6061Al, and 7075Al) that were later buried for 9 months in natural soil containing sulphate-reducing bacteria (SRB). A peptone solution was added to the soil to induce the growth of SRB. Coated and uncoated coupons retrieved after the exposure period displayed distinctly different surface characteristics. Excessive degradation of the surface due to corrosion was observed on the uncoated coupons, while the coated coupons were relatively free of corrosion (Fig. 25), except at edges and corners on the specimen where the coating may have been damaged. No residue was left on the coated coupons after they were washed, indicating that the coating may have anti-fouling characteristics [136].

Figure 25. Surface scans of uncoated (UC) and coated (C) coupons retrieved after exposure to soils containing SRB. Coupons were buried for 9 months in soil obtained from Waimanalo and supplemented with a peptone solution to induce microbial growth. Uncoated coupons suffered significant damage due to corrosion, while the coated coupons were not significantly affected.

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Novel Silicone Ceramer Coatings for Aluminum Protection